This invention relates to digital radio communication systems, and more particularly to efficient adaptive modulation within such systems.
In digital radio communication systems a transmitter transmits a signal at a transmission rate to a receiver through a radio channel having channel characteristics, such as an attenuation. The data is transmitted using transmission parameters, such as a modulation level and a coding rate. The transmission rate depends on the transmission parameters. The transmission parameters are constrained by an acceptable bit error rate and by a signal to interference ratio of the signal, the latter varying in time with the channel characteristics. If the signal to interference ratio decreases, the modulation level must be reduced (for example, from 16-QAM to QPSK) or the coding rate must be improved (for example, from 3/4 to 2/3) in order to maintain the acceptable bit error rate. Either of these changes results in a lower transmission rate.
The communication system can use adaptive modulation to adjust the transmission parameters to accommodate changes in channel characteristics over time. If a change in channel characteristics results in a higher signal to interference ratio, the transmitter can increase the modulation level or decrease the coding rate in order to obtain a higher transmission rate. If a change in channel characteristics results in a lower signal to interference ratio, the transmitter can decrease the modulation level or increase the coding rate to maintain the acceptable bit error rate, albeit at the expense of a lower transmission rate.
In communication systems that implement adaptive modulation, the transmitter and the receiver must be synchronized with respect to the transmission parameters. In current communication systems the receiver determines a channel quality when the receiver receives a frame of data. The receiver may estimate, for example, the signal to interference ratio of the channel. The receiver sends a signal back to the transmitter reporting the channel quality. Using the channel quality report from the receiver, the transmitter calculates optimum transmission parameters which the transmitter will use in its next transmission of data. However, the transmitter must first send the new optimum transmission parameters to the receiver using the previous transmission parameters. The receiver receives the new optimum transmission parameters, interpreting the signal using the previous transmission parameters. When the receiver receives the next transmission of data, the receiver interprets the signal using the new optimum transmission parameters it has just received from the transmitter.
This adaptive modulation scheme requires much signalling between the receiver and the transmitter. For example, if the reported signal to interference ratio has a range of 20 dB and a resolution of 1 dB, then five bits of information are needed to describe the signal to interference ratio. The channel quality must be encoded strongly so that the transmitter will receive the correct report and calculate the transmission parameters appropriate to the quality of the channel. For a coding rate of 1/4, the number of bits which must be sent from the receiver to the transmitter rises to twenty. The transmitter must also signal the optimum transmission parameters to the receiver, which may require a further twenty bits.
In communications systems that make use of multiple antennas for transmission and reception, the transmission parameters may include adaptive antenna and coding parameters. For example, some xe2x80x9csmart antennaxe2x80x9d systems may adaptively adjust their directional patterns towards the remote units. An outline of such systems may be found in the paper by J. H. Winters, xe2x80x9cSmart Antennas for Wireless Systemsxe2x80x9d, IEEE Pers. Commun., vol. 5, no. 1, Feb. 1998, pp 23-27, which is incorporated herein by reference. Similarly, the radio system may make use of the multiple communications channels that exist between transmitters and receivers with multiple antennas. In this case the transmission parameters include both space (across multiple antennas) and time (different time of transmissions) aspects that adapt the transmissions to the multiple propagation environment. An outline of such systems may be found in the paper by A. J. Paulraj and B. C. Ng, xe2x80x9cSpace-time Modems for Wireless Personal Communicationsxe2x80x9d, IEEE Pers. Commun., vol. 5, no. 1, Feb. 1998, pp. 36-48, which is incorporated herein by reference.
In radio communications systems, the transmission parameters may be adaptively adjusted to the radio channel conditions through feedback from the receiver (remote station) to the transmitter (base station). The xe2x80x9clink modexe2x80x9d used by the system will consist of a set of transmission parameters that include some (or all) of: modulation scheme, error control coding scheme, symbol rate, transmission power level, antenna directional parameters, or space-time coding (STC) parameters.
In communication systems employing many sub-carriers, such as one that employs Orthogonal Frequency Division Multiplexing (OFDM), the channel quality will vary with the frequency of each sub-carrier. Information describing the channel quality and the optimum transmission parameters must be transmitted for each sub-carrier. Systems which employ OFDM can use 1000 sub-carriers, and the number of bits that must be transmitted in each direction in order to implement adaptive modulation may be 20000. This is significant overhead, and reduces the efficiency of the communication system.
The present invention provides a method of adapting a current Link Mode (LM) in a radio communication system, a LM being a set of at least one transmission parameter, such as a modulation scheme or a coding rate. The communication system includes a base station which transmits a signal to a remote unit using the current LM. A sequence of allowed LMs is stored at the base station and at the remote unit. At the remote unit a channel quality of the signal, such as a signal to interference ratio, is determined. At the remote unit, a desired LM is determined from amongst the sequence of allowed LMs based on the channel quality. At the remote unit, a sequential difference between the sequential position of the current LM and the sequential position of the desired LM is determined. The remote unit transmits the sequential difference to the base station. The remote unit sets a new current LM to be the desired LM. The base station sets a new current LM to be the desired LM using the sequential difference. The remote unit may also periodically transmit the desired LM to the base station.
The present invention also provides a method of determining a signal to interference ratio of a signal sub-carrier in a communication system. The communication system includes a base station which transmits a pilot signal to a remote unit over a pilot sub-carrier. The pilot signal may be either on or off. The remote unit measures a signal strength of the pilot sub-carrier when the pilot signal is on and measures a signal strength of the pilot sub-carrier when the pilot signal is off, the latter being in effect a measurement of interference in the pilot sub-carrier as there is no pilot signal. The remote unit calculates a ratio of the signal strength when the pilot signal is on to the signal strength when the pilot signal is off.
The method provides improved efficiency of a communication system by reducing the amount of overhead needed to adapt the transmission parameters to changing channel quality.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.